Initial implementation

Move auth logic out of Card API to a shared library, this is largely a lift-and-shift with some documentation and additional tests, with the code being moved out of: https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-api/-/merge_requests/90.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-photos-service/photo-api/-/issues/16.